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The duration calculation was using %, but that's only supported on
duration objects when the arithmetic type supports %, and hence fails
for floats.  Fixed by subtracting off the calculated values instead.

A flake8 configuration is included in setup.cfg and the checks are
executed automatically on Travis:

* Ensures a consistent PEP8 code style
* Does basic linting to prevent possible bugs

Allowed durations and non system clocks to be set from floats.

Add unit tests and documentation for the chrono cast.